Articles | Volume 14, issue 5
Biogeosciences, 14, 1165–1179, 2017
Biogeosciences, 14, 1165–1179, 2017

Research article 13 Mar 2017

Research article | 13 Mar 2017

Diatoms as a paleoproductivity proxy in the NW Iberian coastal upwelling system (NE Atlantic)

Diana Zúñiga1,2, Celia Santos3,4,5, María Froján2, Emilia Salgueiro3,5, Marta M. Rufino3,5,7, Francisco De la Granda6, Francisco G. Figueiras2, Carmen G. Castro2, and Fátima Abrantes3,5 Diana Zúñiga et al.
  • 1University of Vigo, Applied Physics Department, Campus Lagoas Marcosende, 36310, Vigo, Spain
  • 2Consejo Superior de Investigaciones Científicas (CSIC), Instituto de Investigaciones Marinas (IIM), 36208, Vigo, Spain
  • 3Instituto Português do Mar e da Atmosfera (IPMA), Div. Geologia e Georecursos Marinhos, 1495-006, Lisbon, Portugal
  • 4MARUM, Center for Marine Environmental Sciences, University of Bremen, 28359, Bremen, Germany
  • 5CCMAR – Centre of Marine Sciences, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
  • 6Federal Maritime and Hydrographic Agency of Germany, 20359, Hamburg, Germany
  • 7IFREMER, Centre Atlantique (French Research Institute for Exploitation of the Sea), 44311, Nantes, France

Abstract. The objective of the current work is to improve our understanding of how water column diatom's abundance and assemblage composition is seasonally transferred from the photic zone to seafloor sediments. To address this, we used a dataset derived from water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system.

Diatom fluxes (2.2 (±5.6) 106 valves m−2 d−1) represented the majority of the siliceous microorganisms sinking out from the photic zone during all studied years and showed seasonal variability. Contrasting results between water column and sediment trap diatom abundances were found during downwelling periods, as shown by the unexpectedly high diatom export signals when diatom-derived primary production achieved their minimum levels. They were principally related to surface sediment remobilization and intense Minho and Douro river discharge that constitute an additional source of particulate matter to the inner continental shelf. In fact, contributions of allochthonous particles to the sinking material were confirmed by the significant increase of both benthic and freshwater diatoms in the sediment trap assemblage.

In contrast, we found that most of the living diatom species blooming during highly productive upwelling periods were dissolved during sinking, and only those resistant to dissolution and the Chaetoceros and Leptocylindrus spp. resting spores were susceptible to being exported and buried. Furthermore, Chaetoceros spp. dominate during spring–early summer, when persistent northerly winds lead to the upwelling of nutrient-rich waters on the shelf, while Leptocylindrus spp. appear associated with late-summer upwelling relaxation, characterized by water column stratification and nutrient depletion. These findings evidence that the contributions of these diatom genera to the sediment's total marine diatom assemblage should allow for the reconstruction of different past upwelling regimes.

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Short summary
Diatoms are one of the most important primary producers in highly productive coastal regions. Their silicified valves are susceptible to escape from the upper water column and be preserved in the sediment record, and thus are frequently used to reconstruct environmental conditions in the past from sediment cores. Here, we assess how water column diatom’s community in the NW Iberian coastal upwelling system is seasonally transferred from the surface to the seafloor sediments.
Final-revised paper